1. Field of the Invention
The present invention relates to a method and related communication device used in a wireless communication system and more particularly, to a method of reporting radio access technology (RAT) capability in a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as an evolved 3G (third generation) radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile devices, also referred as user equipments (UEs).
A UE equipped with a multi-mode capability supports RATs (Radio Access Technologies) in addition to the LTE RAT and has corresponding RAT capability information. A UE capability transfer procedure used in the LTE system allows the UE to report RAT capability information to the E-UTRAN by sending a UECapabilityInformation message. With the RAT capability information, the E-UTRAN performs resource/mobility management for the UE, e.g. initiating an inter-RAT handover.
When a UE supporting the LTE uses other RATs, such as GSM (Global System for Mobile communications) or UMTS (Universal Mobile Telecommunications System), the UE can be required to report LTE capability to the corresponding radio access network, such as a GERAN (GSM/EDEG Radio Access Network) or a UTRAN (UMTS Terrestrial Radio Access Network). According to the prior art, the UE also needs to send the abovementioned UECapabilityInformation message that is allowed to include capabilities of various RATs including the GSM/UMTS/LTE/CDMA2000 (Code Division Multiple Access 2000). The UE can always send the UECapabilityInformation message including the LTE and other RATs, thereby resulting in a large size of the UECapabilityInformation message. In addition, the capability information of the RATs other than the LTE in the UECapabilityInformation message is unnecessary if the capabilities of other RATs have just been reported through another procedure/message.
For example, the UECapabilityInformation message is sent by a UE accessing a UTRAN for reporting the LTE, CDMA2000, and UMTS capabilities. However, no inter-working between UTRAN and CDMA2000 is specified. Thus, the CDMA2000 capability information is unnecessary. In addition, the UMTS has specified a UE capability transfer procedure for the UE to report the GERAN/UMTS capability. In this situation, the UMTS capability information in the UECapabilityInformation message is unnecessary. In the UMTS system, the UE is able to report all supported RAT capabilities through a RRC CONNECTION SETUP COMPLETE message of a RRC connection establishment procedure. In the prior art, the RRC CONNECTION SETUP COMPLETE message can include the GERAN/UMTS capability information and the UECapabilityInformation message that also includes the GERAN/UMTS capability information in addition to the LTE capability information. As can be seen, duplication of the GERAN/UMTS capability information is made, resulting in a large size of the RRC CONNECTION SETUP COMPLETE message. This downgrades delays completion of RRC connection establishment procedure and thereby downgrades system performance.
In addition, the prior art does not specify how the UE can report the E-UTRA capability to a core network communicating with the UE in a NAS (Non-Access Stratum) level. Without the E-UTRA capability, the core network cannot initiate a handover for the UE if the UE only support the PS domain. For example, if a UE only supports GPRS (General packet radio service) and does not support GSM (for CS services), a handover from the GERAN to the E-UTRAN for the UE cannot be initiated since the UE cannot use NAS messages to send the E-UTRA capability to a SGSN (Serving GPRS Support Node).